1. Field of Invention
This invention relates to seat belt winding devices and more particularly to seat belt winding devices in which two seat belts are wound up on a single shaft one over the other.
2. Prior Art
In conventional seat belt winding devices for protecting passengers during a collision of a motor vehicle, the three-point type seat belt which consists of a lap belt and a shoulder belt have been widely used. In these three-point type seat belt systems, when the passenger is not using the belt or only a part thereof is being used, the unused portions are wound up by means of spring-loaded winding devices, thereby improving the fitness characteristics and housing of the seat belts.
In the prior art three-point seat belt systems, it has been accepted that two separate winding devices must be installed, one for the lap seat belt and one for the shoulder seat belt to insure the best fitness characteristics and to house the belts. However, it has been proposed that a single winding device could wind both seat belts up without impairing the fitness characteristics and housing.
Referring to FIGS. 1 and 2, shown therein is a seat belt device for winding up two seat belts on a single shaft one over the other. In FIGS. 1 and 2, the seat belt winding device includes a lap seat belt 10A and a shoulder seat belt 10B. Lap seat belt 10A and shoulder seat belt 10B are formed in a single belt and extend through hole 12 in tongue plate 11. This device is designed for winding up both the lap seat belt 10A and the shoulder seat belt 10B on a single shaft one over the other which extends through a slit in an anchor 14 which is fixed to the upper side wall of a motor vehicle 12 above the seat belt winding device 15. The winding device 15 is fixed to the lower part of the sidewall of the vehicle 13 and is arranged and configured such that a lap seat belt 10A and shoulder seat belt 10B are both wound upon a single springloaded shaft 16 one over the other. Accordingly, by using a two belts winding type seat belt device 15, when either one of the seat belts is extended out of the winding device 15, the other seat belt is also extended out of the device 15 at the same time. Moreover, the lap seat belt 10A and the shoulder seat belt 10B are always kept at an appropriate length since the two belts can be moved one way or the other. Therefore, this two belts winding device 15 has many desirable characteristics even though it is a simplified device.
Even though this two belts winding type seat belt 15 has many desirable characteristics, it has one weak point. The weak point is the tension applied to the seat belt during extension and retraction. If the tension applied is very weak, the two seat belts are not wound very tightly onto the shaft 16 and therefore the two belts are not wound up on the shaft of equal length.
This slackened seat belt may entangle in the winding device or prevent the function of the locking bar 19 during an emergency. In an extreme case, it is possible that as a result of the slack the seat belt will not work properly and the fit of the seat belt will be impaired when it is fastened.
Therefore, it is an object of the present invention to overcome the deficiencies of the prior art.